1. Field of the Invention
This invention relates in general to electronic relative position sensing devices or devices which indicate the relative position of two elements, one moveable with respect to the other. More particularly, this invention relates to the carriage and housing mechanism of such devices.
2. Description of the Prior Art
In many areas of industry and manufacturing it is important to accurately position one element or device with respect to another. For example, in machining it is important that the tool bit be accurately positioned with respect to the work piece and the table holding the work piece. To allow this positioning to be performed a position sensor is often required. In addition to accurate relative positioning, it is also often necessary to move the work piece or element to and from or between very accurate relative positions.
For example, it might be desired to perform a first machining operation at one relative position and then automatically move the work piece to a second position and perform a second machining operation. Many times it is desired to have great accuracy in the machined work resulting from the first and second position. A position sensor with a reading mechanism therein can be used for automatic performance of such multiple stops. As the accuracy of the reading mechanisms increases it is required that the carriage mechanism for the reader mechanisms also be more precise to hold the reader for smooth, one-dimensional movement. This motion of the reader in the housing must be isolated from out-of-alignment movement in the pieces which move the reader.
Among the devices which desirably use such extremely accurate relative position sensing methods and devices are lathes, milling machines, microscopes, telescopes, industrial robots, cylinders, flight controls, drill presses, production automation equipment, etc.
Among the devices currently available for position sensing are electronic relative position sensors which have scales provided on them and means for electrically reading these scales. The scales can be attached to the lathe table or the like and the electrical readers of the scales can be connected to the lathe bed or the like, or vice versa. As the device moves, the reader electrically senses or determines the relative position between the scale and reader and, thereby, the relative position between the devices to which they are attached.
The carriage mechanism for these devices typically includes a carriage frame which rides within a housing. The frame carries the electrical reader next to the scale which is fixed within the housing. Usually bearings are mounted on the frame and these support the frame for rolling on the scale or on bearing surfaces of the housing. The frame is attached to one of the relative position pieces (such as a bed of a lathe) so that the position of this piece is followed by the reader while the scale and housing are fixed to the other relative position piece (such as the moveable table of the lathe) and follows this piece.
A serious problem with such carriage mechanisms and housings is that the connection between the carriage frame and the relative position element is not sufficiently stable to prevent some play or movement therebetween when the device changes direction. For example, a universal joint-type of connection must allow some movement in order not to be too stiff so as to move the frame off of the bearing surfaces and out of alignment with the scale. This movement in the universal joint causes hysteresis in the position measurement or sensing. Conversely, without a universal joint connection or a connection which allows some movement, out of alignment movement of the reader can destroy the accuracy of the measurement read or even damage the scale or the bearings.
Another problem is that even with a universal joint connection to the carriage frame, tilting moments of the carriage frame occur as the carriage is driven in one or both directions (depending on where the universal joint is located). Similarly, the bearings on the frame must be very carefully balanced and aligned to prevent rocking of the frame caused by differing pressures applied to the bearings. A similar problem is encountered because the pressures on the bearings must be maintained by springs between the frame and the relative position element. These springs can produce different bearing pressures as the carriage moves in different directions. This also causes rocking of the frame, which causes hysteresis in the position measurement or sensing.
Although precision bearings and joints can reduce the above problems, they can not eliminate them. Reduction of these problems by such precision elements and components is not suffient when the reader is capable of extreme accuracy or as described in U.S. patent application No. 217,015, filed July 8, 1988 and entitled Precision Electronic Absolute and Relative Position Sensing Device and Method of Using Same (the disclosure of which is hereby incorporated by reference).
Another problem of these types of carriage mechanisms and housings is that the mounting process for mounting the housing to a lathe or the like can stress or twist the housing causing the carriage frame to ride improperly and inaccurately therein, as well as distorting the scale.